The invention generally relates to biomedical electrodes and relates in particular to biomedical electrodes for detecting localized electrical signals within a subject as well as biomedical electrodes for providing electrical stimulation to a subject.
Conventional disc biomedical electrodes have generally changed little since Hans Berger first recorded the human electroencephalogram (EEG) in 1924. One drawback of conventional EEG methods that are recorded with disc electrodes, is that the procedure lacks high spatial resolution. This is primarily due to the blurring affects of the different conductivities of the volume conductor such as the cerebrospinal fluid, skull, and the scalp. Conventional EEG signals recorded with disc electrodes also have reference electrode problems as idealized references are not available with EEG. Placing the reference at different locations changes the characteristics of the EEG signals.
There are previous reports of concentric ring electrodes. Many such reports however, such as “Exploration du Champ Electrique Precordial a l′ aide de deux Electrodes Circulaires, Concentrique et Rapproches” by V. Fattorusso and J. Tilmant, Arch Mal du Coeur, v. 42, pp 452-455 (1949); “Body Surface Laplacian Mapping in Man” by B. He and R. J. Cohen, IEEE EMBS, v. 13, no. 2, pp. 784-786 (1991); and “Computing the Lead Field of Electrodes with Axial Symmetry” by A. van Oosterom and J. Strackee, Medical & Biological Engineering & Computing, no. 21, pp. 473-481 (1983) only disclose bipolar concentric electrodes. The article “Concentric-Ring Electrode Systems For Non-Invasive Detection of Single Motor Unit Activity” by D. Farina and C. Cescon, IEEE, Transactions in Biomedical Engineering, v. 48, no. 11, pp. 1326-1334 (November 2001) describes various concentric electrodes with up to four rings, but there is no specific description of how the signals were acquired (for example including electronic connectivity) other than the fact that weights are employed. U.S. Patent Application Publication No. 2004/0199237 (to Mills et al.) discloses a method and device for testing whether an electrode is functioning properly by passing small currents between a ring and a disc electrode. U.S. Patent Application Publication No. 2003/0125786 (to Gliner et al.) describes a tripolar concentric electrode for implantable neurostimulation use.
To increase the spatial frequency and selectivity the surface Laplacian has been utilized. Concentric ring electrodes automatically estimate the surface Laplacian significantly better than by processing conventional EEG signals (see “Development of Tri-Polar Concentric Ring Electrode for Acquiring Accurate Laplacian Body Surface Potentials”, by W. Besio, R. Aakula, K. Koka and W. Dai, Annals of Biomedical Engineering, Vol. 34, No. 3, March 2006) and significantly improves the signal-to-noise level in EEG applications, (see “Tri-Polar Concentric Ring Electrode Development for Laplacian Electroencephalography, by W. Besio, R. Aakula, K. Koka and W. Dai, IEEE Transactions on Biomedical Engineering, Vol. 53, No. 5, May 2006), as well as spatial selectivity, and mutual information (see “Improvement of Spatial Selectivity and Decrease of Mutual Information of Tri-Polar Concentric Ring Electrodes”, by K. Koka and W. Besio, Journal of Neuroscience Methods, Vol. 165, pp. 216-222, Jun. 9, 2007). The reference problem is alleviated as well since bipolar differences are taken at closely spaced electrode elements.
Theory has shown that the elements of the electrodes should have equal area to prevent electrode offset potentials due to electrode half-cell potentials (see “Body Surface Laplacian ECG Mapping” by B. He and R. J. Cohen, IEEE Transactions in Biomedical Engineering, v. 39, no. 11, pp. 1179-1191 (1992); and “Medical Instrumentation Application and Design 3rd ed.” by J. Webster, John Wiley & Sons, Inc. (1998)). To use the finite difference approximation of the Laplacian described in “Numerical Methods for Partial Differential Equations” by W. F. Ames, Barnes & Noble, Inc., NY, pp. 15-19 (1969) and the methods of “Difference Formulas for the Surface Laplacian on a Triangular Surface” by G. Huiskamp, J. Computational Physics, v. 95, no. 1, pp. 477-496 (1991) to relate the finite difference method to a concentric ring electrode, the implied theory is that the center disc is a ring whose radius is equal to the ring and the outer ring is the same thickness as the ring of the disc. When the rings are of equal thickness the diameter of the central disc is twice the thickness of the outer ring. “Numerical Methods for Partial Differential Equations” by W. F. Ames, Barnes & Noble, Inc., NY, pp. 15-19 (1969) also states that the spacing between the rings, and disc, should be equal. For example, FIG. 1 shows a prior art concentric ring electrode 10 that includes a disc 12, an inner ring 14 a middle ring 16 and an outer ring 18. As discussed above, conventionally, the thickness of each ring is the same (t) and the diameter of the disc 12 is 2t. The spacing between the disc and each of the rings (which includes a dielectric material 8) is also conventionally t.
Further, an electrode gel (e.g., an electrolyte) has conventionally been used to bridge between electrodes and a cleaned surface of a subject (e.g., the scalp). The spacing required between electrodes may be so small that smearing of the electrolyte (and thus short circuiting of the bioelectric signal) may occur. Additionally, and perhaps most importantly, the application and removal of electrolyte gels is an unpleasant process for the subject, and time consuming for the clinician or care giver. There are also toxicological concerns with electrolyte gels where dermatological responses are common.
To avoid the problems of electrolytes, dry electrodes (not using a gel) have been introduced. With dry electrodes, however, movement artifacts are more prevalent due to the absence of a thick electrolyte layer (as is present in gels, which provides a shock absorber function). The introduction of active electrodes (where buffering/amplification takes place at the electrode site) provides much less emphasis on the skin-electrode impedance. An added concern with dry electrodes is that the large RC constant, which exists at the input of the unity gain amplifiers typically used for this application, prolongs the effect of large artifacts.
There is a need therefore, for an improved biomedical electrode that may be used without the current drawbacks yet may also provide consistent and reliable high resolution signals.